Hybrid organic-inorganic perovskite materials have attracted interest as promising materials for both photovoltaic and optoelectronic applications. Three-dimensional (3D) organic-inorganic hybrid perovskites generally adopt the formula of ABX3, wherein A is an organic cation, B is a metal ion and X is a halide anion. Two-dimensional (2D) organic-inorganic hybrid perovskites generally adopt the formula of A2BX4. These 2D materials are layered structures in which each layer consists of an extended network of corner-sharing metal halide octahedral [MX6]4− and two layers of organic cations on both sides to balance the charge.
Methylammonium lead iodide perovskite, CH3NH3PbI3 (MAPbI3), is a 3D perovskite material emerging as a “super-star” semiconductor for cost-effective photovoltaic (PV) applications. It is a semiconductor with a suitable and direct optical band gap (1.57 eV), a high optical absorption coefficient (α=104-105 cm−1 for hv>1.7 eV), and a long electron/hole diffusion length (a few μm) even in solution-processed polycrystalline thin films, making MAPbI3 advantageous in photovoltaic applications. A variety of methods have been used to prepare MAPbI3 thin films for photovoltaic devices, including spin coating from a MAPbI3 solution, sequential solution deposition, vapor co-evaporation, and vapor-assisted solution conversion. However, these techniques usually produce polycrystalline MAPbI3 perovskite thin films. Crystallinity, shape and size all affect the ability to make use of the MAPbI3 in electronic, optoelectronic and photonic applications.
Formamidinium lead iodide perovskite, CH(NH2)2PbI3 (FAPbI3), is another 3D perovskite which is receiving attention in the photovoltaic research community, although successful incorporation of this material into viable optoelectronic devices other than solar cells has been limited. Use of formamidinium in place of methylammonium leads to a semiconductor with a slightly lower bandgap of 1.47 eV, as well as better temperature and moisture stability.
Similar to 3D methylammonium lead triiodide perovskite, the 2D layered perovskite thin films may be prepared by similar methods. Such 2D perovskites have been used in electroluminescence (EL) devices, scintillation detectors for X-ray radiation, optical microcavities, and exciton or bi-exciton lasing. However, the device performance and photostability of the 2D perovskites has been limited, at least in part due to poor crystal quality.
Semiconductor nanowire (NW) lasers, due to their ultra-compact physical sizes, highly localized coherent output, and efficient waveguiding, are promising building blocks in fully integrated nanoscale photonic and optoelectronic devices. Each NW can serve as waveguide along the axial direction while the two end facets form a Fabry-Perot cavity for optical amplification. Optically pumped lasing has been demonstrated from a number of classic inorganic semiconductor NWs, such as ZnO, GaN, CdS and GaAs with emission from the UV to the near-IR regions . One of the major obstacles limiting potential applications of semiconductor NW lasers is the high threshold carrier density required for lasing. The high lasing threshold means low quantum efficiency; this not only makes key technical advancement, such as electrically driven lasing and integration into optoelectronic devices difficult, but also imposes fundamental limits due to the onset of Auger recombination losses. Despite considerable efforts to improve NW quality using demanding growth conditions that usually require high temperature and high vacuum and core/shell structures to reduce surface recombination, lasing thresholds in NW lasers remain unsatisfactorily high.